Purpose: To assess the long-term stability of gingival grafts placed around dental implants at the time of second surgery uncovering and to further investigate the association between mucosal thickness (MTh) by demographic variables and clinical investigation.Materials and Methods: Fourteen patients with submerged dental implants covered by inadequate keratinized mucosa were studied. The subjects underwent a periimplant plastic surgery (PPS) at the second-stage dental implant surgery and free gingival autograft orsubepithelial connective tissue graft were used according to the patients' clinical situation. Clinical measurement of MTh was assessed by bone sounding with a periodontal probe using customized acrylic stents andthe values were recorded at baseline (day of graft) and at 0.5, 1.5, 4, 12, 24 and 36 months after grafting.Results: At 12 months postoperatively, the mean MTh was 2.89 mm, with a mean additional increase of 1.75 mm when compared with baseline (P=0.0001). No statistically significant differences in MTh were found between the 12- and the 36-month observations (P=0.09). In addition, at 36 months, a thin mucosa was associated with a greater increase in the MTh compared with a thick mucosa (2.14 and 0.64 mm, respectively, P=0.006). Similarly, the mandibular sites were associated with a greater increase in the MTh in comparison with the maxillary sites (2.17 and 0.81 mm, respectively; P=0.02).Conclusions: Within the limitations of this investigation, the data suggest that PPS at the second-stage dental implant surgery could results in additional increases in MTh, especially when it is performed in areas where the mucosa is of a thin biotype.

Currently, it is widely accepted that keratinized mucosa (KM), although not essential to the existence of a tooth and its attachment apparatus, does enhance the long-term survival of the tooth. On a review of the literature, it is evident that the presence of KM is especially important around restorations and prostheses or if the tooth is in a dentition susceptible to periodontal breakdown. [1],[2],[3],[4],[5] However, there has been considerable discussion whether the extent of KM adjacent to implants bear the same significance as to natural teeth. [6],[7],[8],[9],[10],[11],[12],[13],[14],[15] The structure and function of the mucosa that surrounds the implants has been examined, [16] and it was observed that the soft tissue response to plaque develops in a similar manner around natural teeth and dental implants. [17],[18] Longitudinal studies definitively establish that while patient comfort may be enhanced in selected patients with the presence of keratinized gingiva the around implants, its presence is neither necessary for the establishment of osseointegration or for its long-term maintenance. [19]

However, with increasing duration of plaque accumulation, the periimplant mucosa seems to be less-effective in encapsulating the inflammatory lesion. [20] Dental implants are often placed in patients with a history of poor oral hygiene and edentulism, and can have structures and surfaces that are different from those of natural teeth. Thus, they are more susceptible to developing inflammation and bone loss from plaque accumulations or microbial invasion, and the establishment of an adequate amount of gingiva firmly attached to the underlying periosteum and bone has been cited as a goal in implant maintenance. [6]

Nevertheless, little is known about the influence of KM dimension around implants on the hard and soft tissue health. Recent studies showed that mucosal inflammation and plaque accumulation were significantly higher around implants with KM<2 mm, [21] that there was a negative correlation between KM and mucosal recession [22] and that increased width of KM is associated with lower mean alveolar bone loss. [23] More recently, Zigdon and Machtei showed that a thick mucosa (≥1 mm) was associated with lesser mucosal recession compared with a thin mucosa (≤1 mm). [23]

The general belief that mucosal thickness (MTh) and KM width are of special importance in the esthetic zone, where narrow and thin KM may lead to greater mucosal recession, has resulted in the introduction of numerous surgical procedures to increase the width of the gingiva around the dental implants. [6],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37]

The purpose of the present study is to assess the stability of gingival grafts around dental implants and to further investigate the association between MTh and clinical and demographic variables.

Materials and Methods

The subjects eligible for the study were identified from a population of patients attending the Unit of Implantology at the University of Milan School of Dentistry, Italy.

The following inclusion criteria were used: (a) lack of attached KM at the end of the recommended submerged healing period and(b) medically healthy adults. The following exclusion criteria were considered: (a) severely reduced vestibular fornix,(b) pregnancy and lactation,(c) heavy smoking (>10 cigarettes/day),(d) poor oral hygiene and (e) contraindications to periodontal surgery.

Informed written consent was obtained from all patients before they were included in this study.

Data collection

[Figure 1] illustrates the study design and timetable. The same operator performed all the recordings and surgical procedures for all patients. Clinical measurements were recorded at the baseline examination (before graft surgery) and at 0.5, 1.5, 4, 12, 24 and 36 months after graft surgery.

Before the baseline measurement, a customized acrylic stent was fabricated in order to obtain reliable and reproducible measurements of MTh. Starting from alginate impressions, a working cast was fabricated with a wax-up of the proposed restoration and a soft tissue wax-up was then performed with pink wax to develop the ideal soft tissue thickness. An alginate impression was taken to create the working cast for fabrication of a master cast. A custom acrylic stent was then fabricated on the master cast to measure the MTh after grafting positioning and to have controllable and reproducible conditions. The stent had a circular metal sleeve, allowing for passage of a North Carolina probe(UNC 6; Hu-Friedy, Chicago, IL, USA) [Figure 2].

Transgingival probing measurement was assessed after the application of local anesthesia in the buccal gingiva with xylonar spray (Lignocaine 15.0g). Measurements were then rounded up to the nearest millimeter.

Surgery

Before beginning the study, each patient received a full-mouth scaling and polishing and oral hygiene instructions. Preoperative clinic photographs were taken [Figure 3]. A mouth rinse with 15ml of 0.2% chlorhexidine solution (Dentosan; ; Pfizer Consumer Healthcare, Rome, Italy) for 1 min was used before surgery and immediately after the operation.

Figure 3: Clinical preoperative evaluation of the keratinized mucosa at the stage of second surgery unrecovering screw

Because the sample included only patients with poor or absent KM and vestibular fornix maintained, periimplant plastic surgery (PPS) techniques have been used according to the Maiorana and Speroni classification [37][Table 1]. At the time of uncovering of implant and healing screws placement, all patients underwent to the elevation of a split-thickness flap.The preparation of the recipient bed was prepared using a size 15c scalpel blade mounted in a Bard/Parker scalpel handle. The bilaminar technique and free epithelial-connective graft [38],[39],[40],[41] were chosen for performing surgery in order to offer the best esthetic results to the patients [Figure 4], [Figure 5] and [Figure 6]. The donor site was located on the same lateral palate, except for one gingival graft that was obtained from the tuberosity.

To survey soft tissue grafts under the same biological conditions of healing and maturation, all patients had implant-retained restorations 2 months postoperatively.

Study variables, data management and analysis

We used Student's t-tests for paired data to compare the mean MTh measured postoperatively against either MTh at baseline or at 36 months. Unpaired t-tests were used to evaluate the mean keratinization at baseline and at 36 months in relation to gender, age and clinical covariates (mucosal thickness, arch, receiving site, type of performed graft and type of connection fixtureabutment). MTh difference in relation to the same covariates was analyzed with simple and multivariate linear regression models. Statistical analyses were performed using Statistica version 6 and Stata version 10.

Results

Fourteen patients (eightfemales and sixmales) aged 28-58 years (average, 42.2 years) were entered in the study. The mean MTh at baseline was 1.14 mm. We recorded a mean increase in MTh of 4.11 mm at 2 weeks after surgery, followed by a progressive reabsorption, which was faster and almost complete within 8 months (mean increase in MTh at 12 months=1.75 mm; P=0.09 vs. MTh at 36 months).The mean MTh at 36 months was 2.54 mm (P=0.0005 compared to baseline), with a mean gain of 1.40 mm [Table 2] and [Figure 9].

At 36 months after grafting, patients with thin morphotype had a 3.3-times increase of MTh (2.14 mm) in comparison withthick morphotype (0.64 mm; P=0.006) [Table 3]. Patients who received a graft in the lower jaw had an increase in MTh of 2.17 mm, over 2.5-times in comparison withthose operated in the upper jaw (0.81 mm; P=0.02). In multiple regression models, the morphotype remained as the strongest predictor of increase in MTh at 36 months postoperatively (data not shown).

To date, there have been a limited number of studies investigating the relationship between the presence or absence of keratinized tissue and periimplant health in the long-term maintenance of dental implants. Recently, Chung et al. showed that mucosal inflammation and plaque accumulation were significantly higher around implants with KM <2 mm. [21] More recently, Bouri et al. reported that increased width of KM around implants is associated with lower mean alveolar bone loss and improved indices of soft tissue health. [23] Likewise, Zigdon and Machtei investigated the association between the dimensions of KM with clinical and immunological parameters around dental implants. [24] It was observed that a thick mucosa (≥1 mm) was associated with lesser buccal mucosal recession compared with a thin mucosa (<1 mm). These findings are of special importance in the esthetic zone, where narrow and thin KM may lead to greater mucosal recession. Therefore, the periodontal biotype not only affects the natural dentition but also affects the esthetic result in an implant-supported prosthesis as well. In fact, the type of tissue around an implant is essential for the ideal emergence profile and esthetic illusion of a natural tooth [42] and for the long-term health of the implant and its ability to withstand periimplantitis. [6],[43]

In the recent literature, the procedure most indicated to increase the width of attached gingiva seems to be the gingival graft. It is a reliable procedure and graft shrinkage is around 35%. However, it may have some problems, such as the need for a donor area with sufficient dimensions to remove a graft, two surgical areas with a possibility of increasing pain after surgery and a longer time of surgery. [53],[54],[55]

To the best of our knowledge, this is the first study to assess the stability of gingival grafts around dental implants. Eight months after graft, the mean increase in MTh was 1.75 mm (P=0.0001), which decreased to 1.4 mm (P=0.0005) at 36 months postoperatively. The changes from the 12-36 months follow-up were not statistically significant and thus we can state that the graft maintained a dimensional stability 8 months after surgery.

Moreover, in the present study, the stepwise logistic regression analysis was used to assess statistical significance of clinical factors on MTh after PPS. The biotype was the strongest predictor of increased MTh at 36 months postoperatively, with thin biotype showing greater increase of MTh compared with thick biotype [Table 3]. The possible explanation for this phenomenon might be related to the fact that gingival thin phenotype starts with an unfavourable local situation in comparison witha thick one. Recently, Kan et al. showed that the dimensions of periimplant mucosa in the thick biotype were significantly greater as compared with the thin biotype. [52],[55] Therefore, this is no surprise since the thin mucosa benefits more with PPS. [53]

Finally, in the current study, it appears that patients who received graft in the lower jaw had a greater increase of MTh in comparison withthose operated in the upper jaw. Likewise, this difference is probably attributable to the fact that thin morphotype may be more common in mandibular areas. The choice of the technique used for each patient was also related to the treated area and to the patients' morphotype.The patients who underwent uncovering implants in the anterior or esthetic area were treated with a split-thickness flap and connective tissue (CT) placement in order to maintain the color of the treated area unaltered.

Unfortunately, no comparison data are available on this subject and, therefore, these preliminary results should be treated with some caution. To further establish these results, a larger sample size and longer follow-up will be required.

Conclusion

Within the limitations of this investigation, the investigation data suggest that PPS at the secondstage results in additional increases in MTh, especially in the presence of a thick periimplant biotype as compared to a thin biotype. Moreover, stability of the peri-implant mucosal thickness was noted 8 months after the surgical soft tissue correction.